Seats of aircraft, and particularly military aircraft, are frequently equipped with devices generally referred to as energy attenuating apparatuses for absorbing energy generated during a crash or other sudden deceleration so as to reduce the forces applied to a seat occupant and thereby protect the seat occupant against injury. A number of energy attenuating apparatuses work on the principle of plastic deformation of an elongated member. One type of energy attenuating apparatus that has found significant use has an elongated wire, which passes along a nonlinear path between a plurality of rollers. When a force above a certain level acts on the energy attenuating apparatus, the rollers and the wire undergo translation with respect to each other, producing plastic deformation of the wire over a portion of its length as the region of the wire engaging the rollers moves down the length of the wire to dissipate kinetic energy of the seat and the seat occupant. The distance by which the rollers and the wire translate with respect to each other during actuation of the energy attenuating apparatus is referred to as the stroking distance of the energy attenuating apparatus. The stroke depends upon the energy to be dissipated by the energy attenuating apparatus during a crash and the resistance of the wire to deformation.
Existing energy attenuating apparatuses of the type described above employ a constant diameter wire that provides a constant resistance to translation with respect to the rollers over its length. The wire diameter is selected so that the energy attenuating apparatus can provide suitable energy attenuation during a crash for a seat occupant in the 50th weight percentile. While a seat occupant with a weight near the 50th weight percentile can be adequately protected, a light or heavy occupant may not receive sufficient protection. A light seat occupant may suffer dangerously high loads and very little seat stroking, while a heavy seat occupant may experience very light loads and very long seat stroking. Because of the light loads, the heavy seat occupant may require a greater stroking distance than is available in order for the energy attenuating apparatus to dissipate all of the crash energy. Therefore, a seat with a heavy occupant may bottom out before all the energy is attenuated, resulting in very high loads and energy being transmitted to the seat occupant rather than being dissipated by the energy attenuating apparatus.